1. Field of the Invention
In optical sensors in which MOS transistors and a semiconductor light-receiving element are integrated it is easy to integrate the light-receiving element in a one-dimensional or two-dimensional array manner. Such devices are applied in various applications, such as the pickup light-receiving element of an optical disc apparatus, the autofocus light-receiving element of a camera, a facsimile apparatus, a document reading unit of an image scanner, a digital camera, and a video camera. The present invention relates to such an optical sensor that is widely used for consumer products and has a construction where MOS transistors and a semiconductor light-receiving element are integrated.
2. Description of the Related Art
Optical sensors constructed using semiconductor light-receiving elements are broadly divided into CCD-type optical sensors and CMOS-type optical sensors, based on the different methods according to which charges generated at light-receiving sections by the irradiation with light are transmitted to output amplifiers. In the case of the CCD-type optical sensors, the transmission loss of charges generated at the light-receiving sections is suppressed and there occurs less noise during transmission of the charges. Therefore, a high SN ratio is obtained and it is possible to realize high image quality. Consequently, the CCD-type optical sensors are used in various fields, the main one of which is the home video field. The CCD-type optical sensors, however, require a plurality of high voltage power supplies and therefore consume large amounts of electricity, in comparison with ICs and LSIs that are usually used. Also, the method of manufacturing the CCD-type optical sensors greatly differs from that of manufacturing CMOSs constituting integrated circuits such as ICs and LSIs, so that it is difficult to integrate additional functions such as an image processing function.
Recent advancements of portable devices create demands for ICs and LSIs that operate at low power supply voltages, consume less electricity, and have high performance. Similarly, there are created demands for optical sensors that operate at low voltages, consume less electricity, and have integrated additional functions. The CMOS-type optical sensors operate based on MOS transistors constituting ICs and LSIs, so that they can operate at low voltages and reduce power consumption, like ICs and LSIs. Also, the method of manufacturing the CMOS-type optical sensors is the same as that of manufacturing ICs and LSIs, so that it is easy to integrate a high-performance circuit that achieves processing functions. Consequently, the CMOS-type optical sensors receive attention as a technology taking the place of the CCD-type optical sensors.
The CMOS-type optical sensors, however, have a problem that there occurs large noise during the transmission of charges generated at light-receiving sections and therefore image quality is poor in comparison with the CCD-type optical sensors. If a circuit for amplifying and transmitting charges is provided in the vicinity of a light-receiving section in order to reduce such noise, this causes another problem that the area of the light-receiving section becomes relatively small in comparison with a CCD-type optical sensor and therefore the sensitivity of the light-receiving section is decreased.
Also, it is required to manufacture an optical sensor without changing the characteristics of MOS transistors constituting ICs and LSIs for the purpose of integrating an additional circuit. MOS transistors are formed in well regions and the well regions are usually formed using a self-aligned twin well method according to which impurities are implanted into the well regions of one of P-channel MOS transistors and N-channel MOS transistors, thick oxide films are formed through selective oxidation only in the regions in which the impurities have been implanted, and impurities are implanted into the well regions of others of the MOS transistors without using a mask. With the self-aligned twin well method, the well regions of the P-channel MOS transistors always contact the well regions of the N-channel MOS transistors, and the well regions of one of the P-channel MOS transistors and the N-channel MOS transistors formed by implanting impurities exist on a semiconductor substrate. FIG. 5 is a sectional view of the conventional optical sensor.
P-channel MOS transistors 10 and 50, N-channel MOS transistors 20 and 30, and a PN junction diode 1 that is a light-receiving element are formed on a P-type (100) silicon substrate 6. The P-channel MOS transistor 10 includes a gate 12 made of polysilicon and P+ regions 13 that are source and drain regions and are formed in an N-type well region 11. Similarly, the P-channel MOS transistor 50 includes a gate 52 made of polysilicon and P+ regions 53 that are source and drain regions and are formed in an N-type well region 51. Also, the N-channel MOS transistor 20 includes a gate 22 made of polysilicon and N+ regions 23 that are source and drain regions and are formed in a P-type well region 21. Similarly, the N-channel MOS transistor 30 includes a gate 32 made of polysilicon and N+ regions 33 that are source and drain regions and are formed in a P-type well region 31. In the N-type well regions 11 and 51 under the field oxide films 68, N+/xe2x88x92 regions 14 and 54 whose impurity densities are higher than those in the well regions are formed. Also, in the P-type well regions 21 and 31 under the field oxide films, P+/xe2x88x92 regions 24, 25, 34, and 35 whose impurity densities are higher than those in the well regions are formed.
The N-type well regions 11 and 51 have polarities which are different from the polarity of the silicon substrate, and contact the P-type well regions 21 and 31. The diode 1 that is a light-receiving element is constituted by an N-type region 2 and the P-type silicon substrate 6. An N+ region 3 is formed in the N-type region 2 for establishing contact. Field oxide films 8 are formed on the N-type region 2 and light to be received passes through the field oxide films 8 and reaches the diode 1. The N-type region 2 is contact with the P+/xe2x88x92 region 25, the P-type well region 21, the P+/xe2x88x92 region 35 and the P-type well region 31. Consequently, as shown in FIG. 5, even if a PN junction that will function as a light-receiving element is newly formed, the PN junction will contact the well regions of one of the P-channel MOS transistors and the N-channel MOS transistors.
In general, as MOS transistors become finer, there are increased the densities of impurities in the well regions of the MOS transistors. Meanwhile, the capacity of a PN junction is increased in accordance with the increase of the impurity density in and region forming the PN junction. Accordingly, as the packing densities of ICs and LSIs are increased and MOS transistors become finer to accelerate their operation speeds, the capacity of a PN junction serving as a light-receiving element tends to be increased.
Also, the area of a light-receiving element becomes small and the sensitivity thereof is decreased in accordance with the increase of the number of pixels of an optical sensor. Further, the sensitivity of a light-receiving element is proportional to Q/C determined by the amount Q of charges generated by photons reaching the light-receiving element and the capacity c of the PN junction portion of the light-receiving element including a diode. Consequently, the sensitivity of a light-receiving element is decreased in accordance with the increase of the capacity of a PN junction. As a result, if an optical sensor is manufactured according to a manufacturing process for MOS transistors of ICs and LSIs, the sensitivity of a light-receiving element tends to be further decreased as the MOS transistors become finer.
Also, if the impurity densities in the well regions of MOS transistors are changed or heat treatment conditions used, for instance, in an oxidization step during a process for forming the well regions of MOS transistors are changed in order to decrease the impurity density in a PN junction portion that will function as an optical sensor, there is a danger that the characteristics of the MOS transistors are changed. In this case, there is placed a significant burden on the design work of an electronic circuit constructed using MOS transistors.
The present invention has been made in the light of the problems described above and the object of the present invention is to provide an optical sensor that is manufactured without changing the characteristics of MOS transistors constituting ICs and LSIs and has a light-receiving element with a high sensitivity.
The present invention provides an optical sensor in which MOS transistors and a semiconductor light-receiving element are integrated, the light-receiving element includes a PN junction, and charges generated by the irradiation with light are accumulated at the PN junction, characterized in that the PN junction of the light-receiving element is isolated from well regions of the MOS transistors. The present invention also provides a manufacturing method of such an optical sensor. The PN junction of the light-receiving element does not contact with well region of MOS transistor and the impurity of the semiconductor substrate is lower than that of the impurity region or the light-receiving element and well region so that the capacity of the PN junction of the light-receiving element becomes small.
The MOS transistors are P-channel MOS transistors and N-channel MOS transistors, and the well regions of both of the P-channel MOS transistors and the N-channel MOS transistors are formed by implanting impurities into the semiconductor substrate. In the case where the well regions are formed using a self-aligned twin well method, there is obtained an optical sensor in which MOS transistors and a semiconductor light receiving element are integrated, the light-receiving element includes a PN junction, charges generated by the irradiation with light are accumulated at the PN junction, and the PN junction of the light-receiving element is separated from the well regions of the MOS transistors. In this case, the optical sensor is characterized in that well regions of both of P-channel MOS transistors and N-channel MOS transistors are formed by implanting impurities into a semiconductor substrate, and the well regions of the P-channel MOS transistors contact the well regions of the N-channel MOS transistors.
The stated optical sensor is obtained by implanting impurities to form the well regions of one of the P-channel MOS transistors and the N-channel MOS transistors; selectively oxidizing areas above the well regions of the MOS transistors into which the impurities have been implanted; masking an area in which the light-receiving element and a region around the light-receiving element are to be formed with a photoresist; and implanting impurities to form the well regions of the other MOS transistors, thereby forming the well regions of the P-channel MOS transistors and the N-channel MOS transistors. The optical sensor manufactured in this manner has a construction where the PN junction of the light-receiving element is isolated from well regions of the MOS transistors, well regions of both of the P-channel MOS transistors and the N-channel MOS transistors are formed by implanting impurities into a semiconductor substrate, and the well regions of the P-channel MOS transistors contact the well regions of the N-channel MOS transistors.
The stated optical sensor is also obtained by implanting impurities to form the well regions of one of the P-channel MOS transistors and the N-channel MOS transistors; selectively oxidizing areas above the well regions of the MOS transistors into which the impurities have been implanted and an area in which the light-receiving element and a region around the light-receiving element are to be formed; and implanting impurities to form the well regions of the other MOS transistors without using a mask, thereby forming the well regions of the P-channel MOS transistors and the N-channel MOS transistors. The well regions of the P-channel MOS transistors and the N-channel MOS transistors are formed in the manner described above. The optical sensor manufactured in this manner has a construction where the PN junction of the light-receiving element is isolated from well regions of the MOS transistors, well regions of both of the P-channel MOS transistors and the N-channel MOS transistors are formed by implanting impurities into a semiconductor substrate, and the well regions of the P-channel MOS transistors contact the well regions of the N-channel MOS transistors.